Toilet / black tank clog?

[Jane]

My question is mainly how do I unclog my toilet but I’ll give the history below in case the background info is helpful.

I bought a used 2018 Vintage Cruiser 23RSS last Dec. I discovered the sensor was reading 2/3 full but the prior owner said she never used it. She had bought it from a dealership less than a year prior. The dealership claimed they emptied the tank but apparently they didn’t. The former owner paid to have a port-o-potty service come to my home and empty the tank. Not much came out and the tank read 1/3 full after. I sprayed it out with the built in feature and the water ran freely and clear. I didn’t begin using the tank until recently. The sensor didn’t seem reliable so I was guessing on when to dump. Not much solids seemed to be passing through but liquids were. Then, liquid stopped passing through. I filled the tank with water and used Unique tank cleaner. That seemed to help the liquid once again flow out of the tank but I’m not sure if it digested everything in the tank or not. When I open the toilet valve I see liquid and some floating bits so I know the toilet is still clogged. My questions are what should I use to unclog the toilet and how do I know if there’s still a bunch of solids in the tank that aren’t coming out? The sensor now reads empty when I drain the tank and climbs as I add water using the built in sprayer so it seems accurate unless there’s a pile in there that isn’t in contact with the sensors.